docs: def to_cache(self, pref):
          """
          Update/create the cache value for the given preference model instance
          """
          key = self.get_cache_key(pref.section, pref.name)
          value = pref.raw_value
          if value is None or value == '':
              # some cache backends refuse to cache None or empty values
              # resulting in more DB queries, so we cache an arbitrary value
              # to ensure the cache is hot (even with empty values)
              value = preferences_settings.CACHE_NONE_VALUE
          self.cache.set(key, value, None)